Alex,
In my opinion, this is the most exciting underwater photography news in many, many years. Ever since I sold my Nikonos cameras, including the 15mm and started using SLRs in 1990 I have struggled with sharpness when using wide angle lenses.

Many years ago I read an article written by one of the original members of the BSoUP. Back in the 60's, they had to make a lot of the kit themselves, because there was not much that could be bought. The corrector port was one of the things that was popular to make. They made them of acrylic material that was shaped (in a lathe, I presume) and then polished up. But the corrector was soon taken over by the dome ports as it was much easier to make.

Seing that the corrector is close to impossible to find, the only alternative appears to make one. The design is described well in many books. And its not new. Introduced by Gallileo Gallilei. The corrector uses the principle in the reversed way. Also used for optical viewfinders for rangefinder cameras.

From what I have been able to find out the formula for a Reverse Galileo telescope is Fp + Fn = d, where Fp is the focal lenght of the convex lens, Fn the focal lenght for the plano-concave lens, and d is the distance between them. The Rebikoff corrector will correct the magnification of 1.33 caused by the front port by adding a 0.75 magnification. Fn = 0.75 x Fp. I have attempted to calculate, using a +4 close-up lens (the Fp) as a starting point. The positive lens (a given) focal lenght is 25 cm. The plano-concave lens is 18.75 cm. The distance between the two is 62.5mm.
Do you know the focal lenghts for the two lenses involved on your corrector? And the distance between them? Does my figures make sense when compared to the real thing?